U.S. patent application number 13/553743 was filed with the patent office on 2013-04-11 for negative active material and lithium battery containing the negative active material.
The applicant listed for this patent is Yu-Jeong Cho, Ui-Song Do, Jae-Myung Kim, So-Ra Lee, Su-Kyung Lee, Chang-Su Shin, Ha-Na Yoo. Invention is credited to Yu-Jeong Cho, Ui-Song Do, Jae-Myung Kim, So-Ra Lee, Su-Kyung Lee, Chang-Su Shin, Ha-Na Yoo.
Application Number | 20130089784 13/553743 |
Document ID | / |
Family ID | 47018858 |
Filed Date | 2013-04-11 |
United States Patent
Application |
20130089784 |
Kind Code |
A1 |
Cho; Yu-Jeong ; et
al. |
April 11, 2013 |
NEGATIVE ACTIVE MATERIAL AND LITHIUM BATTERY CONTAINING THE
NEGATIVE ACTIVE MATERIAL
Abstract
A negative active material and a lithium battery including the
negative active material. The negative active material includes
primary particles, each including: a crystalline carbonaceous core
having a surface on which silicon-based nanowires are disposed; and
an amorphous carbonaceous coating layer that is coated on the
crystalline carbonaceous core so as not to expose at least a
portion of the silicon-based nanowires. Due to the inclusion of the
primary particles, an expansion ratio is controlled and
conductivity is provided and thus, a formed lithium battery
including the negative active material may have improved
charge-discharge efficiency and cycle lifespan characteristics.
Inventors: |
Cho; Yu-Jeong; (Yongin-si,
KR) ; Lee; So-Ra; (Yongin-si, KR) ; Yoo;
Ha-Na; (Yongin-si, KR) ; Do; Ui-Song;
(Yongin-si, KR) ; Shin; Chang-Su; (Yongin-si,
KR) ; Lee; Su-Kyung; (Yongin-si, KR) ; Kim;
Jae-Myung; (Yongin-si, KR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Cho; Yu-Jeong
Lee; So-Ra
Yoo; Ha-Na
Do; Ui-Song
Shin; Chang-Su
Lee; Su-Kyung
Kim; Jae-Myung |
Yongin-si
Yongin-si
Yongin-si
Yongin-si
Yongin-si
Yongin-si
Yongin-si |
|
KR
KR
KR
KR
KR
KR
KR |
|
|
Family ID: |
47018858 |
Appl. No.: |
13/553743 |
Filed: |
July 19, 2012 |
Current U.S.
Class: |
429/213 ;
429/217; 429/219; 429/220; 429/221; 429/222; 429/223; 429/224;
429/226; 429/229; 429/231.5; 429/231.6; 429/231.8; 977/734;
977/742; 977/948 |
Current CPC
Class: |
Y02E 60/10 20130101;
H01M 4/366 20130101; H01M 4/133 20130101; H01M 4/622 20130101; H01M
4/587 20130101; H01M 10/0525 20130101; H01M 4/134 20130101; H01M
4/485 20130101; H01M 4/386 20130101; H01M 4/623 20130101; H01M
4/625 20130101 |
Class at
Publication: |
429/213 ;
429/231.8; 429/231.6; 429/226; 429/231.5; 429/229; 429/222;
429/224; 429/223; 429/221; 429/220; 429/219; 429/217; 977/948;
977/734; 977/742 |
International
Class: |
H01M 4/583 20100101
H01M004/583; H01M 4/40 20060101 H01M004/40; H01M 4/46 20060101
H01M004/46; H01M 4/42 20060101 H01M004/42; H01M 4/44 20060101
H01M004/44; H01M 4/62 20060101 H01M004/62; H01M 4/60 20060101
H01M004/60; H01M 4/38 20060101 H01M004/38 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 5, 2011 |
KR |
10-2011-0101437 |
Claims
1. A negative active material comprising a primary particle, the
primary particle comprising: a crystalline carbonaceous core with
silicon-based nanowires on a surface thereof; and an amorphous
carbonaceous coating layer coated on the crystalline carbonaceous
core so as not to expose at least a portion of the silicon-based
nanowires.
2. The negative active material of claim 1, wherein at least 50 vol
% of the silicon-based nanowires is embedded in the amorphous
carbonaceous coating layer.
3. The negative active material of claim 1, wherein a thickness of
the amorphous carbonaceous coating layer is in a range of about 0.1
to about 10 .mu.m.
4. The negative active material of claim 1, wherein a D/G ratio of
the amorphous carbonaceous coating layer is 0.31 or more, and
wherein the D/G ratio is a ratio of a D (defect) band peak
intensity area with respect to a G (graphite) band peak intensity
area in a Raman spectrum.
5. The negative active material of claim 1, wherein the amorphous
carbonaceous coating layer comprises an amorphous carbon selected
from the group consisting of soft carbon (cold calcination carbon),
hard carbon, pitch carbide, mesophase carbide, calcined corks, and
combinations thereof.
6. The negative active material of claim 1, wherein an amount of
the amorphous carbonaceous coating layer is in a range of about 0.1
to about 30 wt % based on the primary particle.
7. The negative active material of claim 1, wherein the crystalline
carbonaceous core has a circularity of about 0.2 to about 1.
8. The negative active material of claim 1, wherein a D/G ratio of
the crystalline carbonaceous core is 0.3 or less, and wherein the
D/G ratio is a ratio of a D (defect) band peak intensity area with
respect to a G (graphite) band peak intensity area in a Raman
spectrum.
9. The negative active material of claim 1, wherein the crystalline
carbonaceous core comprises at least one selected from the group
consisting of natural graphite, artificial graphite, expandable
graphite, graphene, carbon black, and fullerene soot.
10. The negative active material of claim 1, wherein an average
particle diameter of the crystalline carbonaceous core is in a
range of about 1 to about 30 .mu.m.
11. The negative active material of claim 1, wherein the
silicon-based nanowires comprise at least one selected from the
group consisting of Si, SiOx (0<x.ltoreq.2), and Si--Z alloys
(where Z is an alkali metal, an alkali earth metal, a Group 13
element, a Group 14 element, a transition metal, a rare earth
element, or a combination thereof and is not Si).
12. The negative active material of claim 1, wherein the
silicon-based nanowires are Si nanowires.
13. The negative active material of claim 1, wherein each of the
silicon-based nanowires independently has a diameter of about 10 to
about 500 nm and a length of about 0.1 to about 100 .mu.m.
14. The negative active material of claim 1, wherein the
silicon-based nanowires are directly grown on the crystalline
carbonaceous core.
15. The negative active material of claim 14, wherein the
silicon-based nanowires are grown in the presence or absence of at
least one metal catalyst selected from the group consisting of Pt,
Fe, Ni, Co, Au, Ag, Cu, Zn, and Cd.
16. The negative active material of claim 1, wherein based on a
total amount of the crystalline carbonaceous core and the
silicon-based nanowires, an amount of the crystalline carbonaceous
core is in a range of about 60 to about 99 wt % and an amount of
the silicon-based nanowires is in a range of about 1 to about 40 wt
%.
17. The negative active material of claim 1, wherein the negative
active material further comprises a carbonaceous particle
comprising at least one selected from the group consisting of
natural graphite, artificial graphite, expandable graphite,
graphene, carbon black, fullerene soot, carbon nanotubes, and
carbon fiber.
18. The negative active material of claim 17, wherein the
carbonaceous particle is in a spherical, tabular, fibrous, tubular,
or powder form.
19. A lithium battery comprising: a negative electrode comprising
the negative active material of claim 1; a positive electrode
facing the negative electrode; and an electrolyte between the
negative electrode and the positive electrode.
20. The lithium battery of claim 19, wherein the negative electrode
further comprises at least one binder selected from the group
consisting of polyvinylidenefluoride, polyvinylidenechloride,
polybenzimidazole, polyimide, polyvinylacetate, polyacrylonitrile,
polyvinylalcohol, carboxymethylcellulose (CMC), starch,
hydroxypropylcellulose, regenerated cellulose,
polyvinylpyrrolidone, tetrafluoroethylene, polyethylene,
polypropylene, polystyrene, polymethylmethacrylate, polyaniline,
acrylonitrilebutadienestyrene, phenol resin, epoxy resin,
polyethylenetelethphalate, polytetrafluoroethylene,
polyphenylsulfide, polyamideimide, polyetherimide,
polyethylenesulfone, polyamide, polyacetal, polyphenyleneoxide,
polybutylenetelephthalate, ethylene-propylene-diene terpolymer
(EPDM), sulfonated EPDM, styrene butadiene rubber, and a fluoride
rubber.
21. The lithium battery of claim 20, wherein an amount of the
binder is in a range of about 1 to about 50 parts by weight based
on 100 parts by weight of the negative active material.
22. The lithium battery of claim 19, wherein the negative electrode
further comprises at least one conductive agent selected from the
group consisting of carbon black, acetylene black, ketjen black,
carbon fiber, copper, nickel, aluminum, silver, and a conductive
polymer.
Description
RELATED APPLICATIONS
[0001] This application claims priority to and the benefit of
Korean Patent Application No. 10-2011-0101437, filed on Oct. 5,
2011, in the Korean Intellectual Property Office, the disclosure of
which is incorporated herein in its entirety by reference.
BACKGROUND
[0002] 1. Field
[0003] One or more embodiments of the present invention relate to a
negative active material and a lithium battery including the
negative active material.
[0004] 2. Description of the Related Art
[0005] Lithium secondary batteries used in portable electronic
devices for information communication (such as PDAs, mobile phones,
or notebook computers), electric bicycles, electric vehicles, or
the like have a discharge voltage that is at least twice as high as
that of a conventional battery and thus have high energy
density.
[0006] Lithium secondary batteries generate electric energy by
oxidation and reduction reactions occurring when lithium ions are
intercalated into or deintercalated from a positive electrode and a
negative electrode, each including an active material that enables
intercalation and deintercalation of lithium ions, with an organic
electrolytic solution or a polymer electrolytic solution interposed
between the positive electrode and the negative electrode.
[0007] As a positive active material for lithium secondary
batteries, for example, an oxide that includes lithium and a
transition metal and has a structure enabling intercalation of
lithium ions may be used. Examples of such an oxide are a lithium
cobalt oxide (LiCoO.sub.2), a lithium nickel oxide (LiNiO.sub.2), a
lithium nickel cobalt manganese oxide (Li[NiCoMn]O.sub.2 or
Li[Ni.sub.1-x-yCo.sub.xM.sub.y]O.sub.2), etc.
[0008] As a negative active material, a carbonaceous base material
and a non-carbonaceous base material, which enable intercalation or
deintercalation of lithium ions, are used and studies thereon have
been continuously performed. Examples of a carbonaceous base
material are artificial graphite, natural graphite, and hard
carbon. An example of a non-carbonaceous base material is Si.
[0009] A non-carbonaceous base material has a very high capacity
that is 10 times greater than that of graphite. However, due to a
volumetric expansion and contraction during charging and
discharging, capacity retention ratio, charge/discharge efficiency,
and lifetime (lifespan) characteristics thereof may be degraded.
Accordingly, there is a need to develop a high performance negative
active material with improved efficiency and lifespan
characteristics.
SUMMARY
[0010] An aspect of one or more embodiments of the present
invention is directed toward a negative active material with
improved capacity characteristics and cycle lifespan
characteristics.
[0011] An aspect of one or more embodiments of the present
invention is directed toward a lithium battery including the
negative active material.
[0012] Additional aspects will be set forth in part in the
description which follows and, in part, will be apparent from the
description, or may be learned by practice of the presented
embodiments.
[0013] According to one or more embodiments of the present
invention, a negative active material includes a primary particle.
The primary particle includes: a crystalline carbonaceous core with
silicon-based nanowires disposed on a surface thereof; and an
amorphous carbonaceous coating layer that is coated on the
crystalline carbonaceous core so as not to expose at least a
portion of the silicon-based nanowires.
[0014] According to an embodiment of the present invention, at
least 50 vol % of the silicon-based nanowires may be embedded in
the amorphous carbonaceous coating layer.
[0015] According to one or more embodiments of the present
invention, a thickness of the amorphous carbonaceous coating layer
may be in a range of about 0.1 to about 10 .mu.m.
[0016] According to one or more embodiments of the present
invention, a D/G ratio of the amorphous carbonaceous coating layer
may be 0.31 or more, wherein the D/G ratio is a ratio of a D
(defect) band peak intensity area with respect to a G (graphite)
band peak intensity area in a Raman spectrum.
[0017] According to one or more embodiments of the present
invention, the amorphous carbonaceous coating layer may include an
amorphous carbon selected from the group consisting of soft carbon
(cold calcination carbon), hard carbon, pitch carbide, mesophase
carbide, calcined corks, and combinations thereof.
[0018] According to one or more embodiments of the present
invention, an amount of the amorphous carbonaceous coating layer
may be in a range of about 0.1 to about 30 wt % based on the
primary particle.
[0019] According to an embodiment of the present invention, the
crystalline carbonaceous core may have a circularity of about 0.2
to about 1. For example, the circularity may be in a range of about
0.7 to about 1, or about 0.8 to about 1, or about 0.9 to about
1.
[0020] According to an embodiment of the present invention, the
carbonaceous material may include a pore or pores therein, and a
porosity thereof may be in a range of about 5 to about 30%.
[0021] According to an embodiment of the present invention, a D/G
ratio of the crystalline carbonaceous core may be 0.3 or less,
wherein the D/G ratio is a ratio of a D (defect) band peak
intensity area with respect to a G (graphite) band peak intensity
area in a Raman spectrum.
[0022] The crystalline carbonaceous core may include at least one
of natural graphite, artificial graphite, expandable graphite,
graphene, carbon black, and fullerene soot.
[0023] According to an embodiment of the present invention, an
average particle diameter of the crystalline carbonaceous core may
be in a range of about 1 to about 30 .mu.m.
[0024] According to an embodiment of the present invention, the
silicon-based nanowires may include at least one of Si, SiOx
(0<x.ltoreq.52), and Si--Z alloys (where Z is an alkali metal,
an alkali earth metal, a Group 13 element, a Group 14 element, a
transition metal, a rare earth element, or a combination thereof
and is not Si). For example, the silicon-based nanowires may be Si
nanowires.
[0025] According to an embodiment of the present invention, each of
the silicon-based nanowires independently may have a diameter of
about 10 to about 500 nm and a length of about 0.1 to about 100
.mu.m.
[0026] According to an embodiment of the present invention, the
silicon-based nanowires may be directly grown on the crystalline
carbonaceous core. In this regard, the silicon-based nanowires may
be grown in the presence or absence of at least one metal catalyst
selected from the group consisting of Pt, Fe, Ni, Co, Au, Ag, Cu,
Zn, and Cd.
[0027] According to an embodiment of the present invention, based
on a total amount of the crystalline carbonaceous core and the
silicon-based nanowires, an amount of the crystalline carbonaceous
core may be in a range of about 60 to about 99 wt % and an amount
of the silicon-based nanowires may be in a range of about 1 to
about 40 wt %.
[0028] According to an embodiment of the present invention, the
negative active material may further include a carbonaceous
particle including at least one of natural graphite, artificial
graphite, expandable graphite, graphene, carbon black, fullerene
soot, carbon nanotubes, and carbon fiber. Herein, the carbonaceous
particle may be in a spherical, tabular, fibrous, tubular, or
powder form.
[0029] According to one or more embodiments of the present
invention, a lithium battery includes: a negative electrode
including the negative active material; a positive electrode that
is disposed facing the negative electrode; and an electrolyte
disposed between the negative electrode and the positive
electrode.
[0030] The negative active material included in the negative
electrode may be the same as described above.
[0031] According to an embodiment of the present invention, the
negative electrode may further include at least one binder selected
from the group consisting of polyvinylidenefluoride,
polyvinylidenechloride, polybenzimidazole, polyimide,
polyvinylacetate, polyacrylonitrile, polyvinylalcohol,
carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,
regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,
polyethylene, polypropylene, polystyrene, polymethylmethacrylate,
polyaniline, acrylonitrilebutadienestyrene, phenol resin, epoxy
resin, polyethylenetelethphalate, polytetrafluoroethylene,
polyphenylsulfide, polyamideimide, polyetherimide,
polyethylenesulfone, polyamide, polyacetal, polyphenyleneoxide,
polybutylenetelephthalate, ethylene-propylene-diene terpolymer
(EPDM), sulfonated EPDM, styrene butadiene rubber, and a fluoride
rubber. An amount of the binder may be in a range of about 1 to
about 50 parts by weight based on 100 parts by weight of the
negative active material. For example, the amount of the binder may
be in a range of 1 to 30 parts by weight, 1 to 20 parts by weight,
or 1 to 15 parts by weight, based on 100 parts by weight of the
negative active material.
[0032] The negative electrode may further include at least one
conductive agent of carbon black, acetylene black, ketjen black,
carbon fiber, copper, nickel, aluminum, silver, and a conductive
polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] These and/or other aspects will become apparent and more
readily appreciated from the following description of the
embodiments, taken in conjunction with the accompanying drawings in
which:
[0034] FIG. 1 is a schematic view of a primary particle included in
a negative active material according to an embodiment of the
present invention;
[0035] FIG. 2 is a schematic view of a lithium battery according to
an embodiment of the present invention;
[0036] FIGS. 3A and 3B show field emission scanning electron
microscope (FE-SEM) images of a cross-section of a negative active
material used in manufacturing a coin cell according to Example
1;
[0037] FIGS. 4A and 4B show FE-SEM images of a negative active
material used in manufacturing a coin cell according to Comparative
Example 1;
[0038] FIG. 5 shows Raman spectrum analysis results of a negative
active material used in the coin cell of Example 1;
[0039] FIG. 6 shows particle size distribution measurement results
of negative active materials used in manufacturing coin cells
according to Examples 1-4 and Comparative Example 1;
[0040] FIG. 7 shows electric conductivity measurement results of
the coin cells of Example 1 and Comparative Example 1;
[0041] FIG. 8 shows pH measurement results of negative electrodes
of the coin cells of Examples 1-4 and Comparative Example 1;
[0042] FIG. 9 shows a result of measuring a volumetric expansion
ratio of a negative electrode when the coin cells of Examples 1-4
and Comparative Example 1 are charged and discharged;
[0043] FIGS. 10A and 10B show graphs of a charge-discharge
efficiency (CDE) of the coin cells of Examples 1-4 and Comparative
Example 1; and
[0044] FIGS. 11A and 11B show graphs of a capacity retention ratio
(CRR) of the coin cells of Examples 1-4 and Comparative Example
1.
DETAILED DESCRIPTION
[0045] Reference will now be made in detail to embodiments,
examples of which are illustrated in the accompanying drawings,
wherein like reference numerals refer to like elements throughout.
In this regard, the present embodiments may have different forms
and should not be construed as being limited to the descriptions
set forth herein. Accordingly, the embodiments are merely described
below, by referring to the figures, to explain aspects of the
present description. Expressions such as "at least one of," when
preceding a list of elements, modify the entire list of elements
and do not modify the individual elements of the list.
[0046] Hereinafter, one or more embodiments of the present
invention are described in more detail.
[0047] A negative active material according to an embodiment of the
present invention includes a primary particle including a
crystalline carbonaceous core with silicon-based nanowires disposed
on a surface thereof, and an amorphous carbonaceous coating layer
that is coated on the crystalline carbonaceous core so as not to
expose at least a portion of the silicon-based nanowires.
[0048] FIG. 1 is a schematic view of a primary particle 100
included in a negative active material according to an embodiment
of the present invention. Referring to FIG. 1, the primary particle
100 of the negative active material includes a crystalline
carbonaceous core 110 with silicon-based nanowires 120 disposed on
a surface thereof and an amorphous carbonaceous coating layer 130
coated on the crystalline carbonaceous core 110 so as not to expose
at least a portion of the silicon-based nanowires 120.
[0049] The term "carbonaceous" included in the crystalline
carbonaceous core 110 refers to inclusion of at least about 50 wt %
of carbon. For example, the crystalline carbonaceous core may
include at least about 60 wt %, 70 wt %, 80 wt %, or 90 wt % of
carbon, or may include only 100 wt % of carbon.
[0050] Also, the term "crystalline" refers to inclusion of at least
about 50 wt % of a hexagonal crystal lattice structure in which a
carbon atom that forms a sp.sup.2 hybrid orbital is covalently
bonded to three other carbon atoms. For example, the crystalline
carbonaceous core 110 may include carbon having about 60 wt %,
about 70 wt %, about 80 wt %, or about 90 wt % of the hexagonal
crystal lattice structure, or may include only carbon having about
100 wt % of the hexagonal crystal lattice structure. The hexagonal
crystal lattice structure may form a single- or multi-layer
structure, or based on a 2-dimensional shape, may have various
deformation shapes, such as a curved shape, a curled shape, a
partially defected shape, or the like. Also, many hexagonal crystal
lattice structures may be connected to form a soccer ball shape. A
crystal structure of the crystalline carbonaceous core 110 is not
limited as long as lithium ions are reversibly intercalated or
deintercalated during charging and discharging. For example, a
plane interval (d002) of a (002) plane due to X-ray diffraction of
the crystalline carbonaceous core 110 may be equal to or greater
than 0.333 nm and less than 0.339 nm, for example, equal to or
greater than 0.335 nm and less than 0.339 nm, or equal to or
greater than 0.337 nm and equal to or less than 0.338 nm.
[0051] According to an embodiment of the present invention, the
crystalline carbonaceous core 110 may include natural graphite,
artificial graphite, expandable graphite, graphene, carbon black,
fullerene soot, or a combination thereof, but examples thereof is
not limited thereto. Natural graphite is graphite that is naturally
formed, and examples thereof are flake graphite, high crystalline
graphite, microcrystalline, cryptocrystalline, amorphous graphite,
etc. Artificial graphite is graphite that is artificially
synthesized, and is formed by heating amorphous carbon at high
temperature, and examples thereof are primary or electrographite,
secondary graphite, graphite fiber, etc. Expandable graphite is
graphite that is formed by intercalating a chemical material, such
as an acid or alkali, between graphite layers, followed by heating
to swell a vertical layer of a molecular structure. Graphene refers
to a single layer of graphite. Carbon black is a crystalline
material that has less regular structure than graphite, and when
carbon black is heated at a temperature of about 3,000.degree. C.
for a long period of time, the carbon black may turn into graphite.
Fullerene soot refers to a carbon mixture including at least 3 wt %
of fullerene that is a polyhedron bundle that is composed of 60 or
more carbon atoms. The carbonaceous core may include one of these
crystalline carbonaceous materials or a combination of two or more
thereof. For example, natural graphite may be used because an
assembly density is easily increased when manufacturing a negative
electrode.
[0052] According to an embodiment of the present invention, a D/G
ratio of the crystalline carbonaceous core 110 may be 0.3 or less,
wherein the D/G ratio is a ratio of a D (defect) band peak
intensity area with respect to a G (graphite) band peak intensity
area in a Raman spectrum. For example, in the Raman spectrum, the
D/G ratio of the crystalline carbonaceous core 110 may be in a
range of about 0.1 to about 0.3. In one embodiment, if the D/G
ratio is equal to or less than 0.3, the carbonaceous core 110 has
crystallinity, and thus, an irreversible reaction of lithium ions
is reduced or minimized during charging and discharging and a
reversible efficiency may be increased.
[0053] According to an embodiment of the present invention, the
crystalline carbonaceous core 110 may be spherical. The term
"spherical" used herein refers to a case in which at least a
portion of the carbonaceous core 110 has a gently or sharply curved
external shape. The carbonaceous base material may have a complete
spherical shape, an incomplete spherical shape, or an oval shape.
It may further have an uneven surface.
[0054] A degree of roundness of the carbonaceous core 110 may be
confirmed by measuring a circularity thereof. Circularity refers to
a measurement value indicating how much the measured shape differs
from a complete circle and has a range of 0 to 1. Thus, if the
circularity is closer to 1, the measured shape is more circular.
According to an embodiment of the present invention, a circularity
of the carbonaceous core 110 may be in a range of about 0.2 to
about 1, or about 0.7 to about 1, or about 0.8 to about 1, or about
0.9 to about 1.
[0055] The spherical carbonaceous core 110 may contribute to
determining the shape of a primary particle, and compared to a
tabular, plate-shaped or lump-shaped carbonaceous core, the
carbonaceous core 110 is not orientated in a particular direction
during pressing (press-molding), and is suitable for high-rate
discharge characteristics, low-temperature characteristics, or the
like. Also, a specific surface area of the carbonaceous core 110 is
reduced and thus reactivity with an electrolytic solution is
decreased. Thus, a formed lithium battery has improved cyclic
characteristics.
[0056] For example, such a spherical crystalline carbonaceous core
110 may be prepared by performing a spheroidizing treatment of a
crystalline carbonaceous material, such as natural graphite,
artificial graphite, expandable graphite, graphene, carbon black,
fullerene soot, etc. For example, a spherical carbonaceous core
obtained by a spheroidizing treatment of graphite may have a
microstructure, in which layered graphite may be gently or sharply
curved, or may have a microstructure that is composed of a
plurality of gently or sharply curved graphite scales or a
plurality of graphite thin films.
[0057] According to an embodiment of the present invention, when
the carbonaceous core 110 is formed in a spherical shape through
the spheroidizing treatment, the carbonaceous core 110 may have a
pore or pores therein. The pore present inside the carbonaceous
core 110 may contribute to a decrease in volumetric expansion of
silicon-based nanowires during charging and discharging. According
to an embodiment of the present invention, the carbonaceous core
110 may have a porosity of about 5 to about 30%, for example, about
10 to about 20%, based on a total volume of the carbonaceous
core.
[0058] An average particle size of the carbonaceous core 110 may
not be limited. However, if the average particle size of the
carbonaceous core 110 is too small, reactivity with an electrolytic
solution is too high and thus cyclic characteristics of a formed
lithium battery may be degraded. On the other hand, if the average
particle size of the carbonaceous core 110 is too large, dispersion
stability in preparing a negative electrode slurry is decreased and
a formed negative electrode may have a rough surface. For example,
an average particle diameter of the carbonaceous core 110 may be in
a range of about 1 to about 30 .mu.m. For example, the average
particle diameter of the carbonaceous core 110 may be in a range of
about 5 to about 25 .mu.m, for example, about 10 to about 20
.mu.m.
[0059] The carbonaceous core 110 may function as a support for
fixing the silicon-based nanowires 120 and may also suppress a
volumetric change of the silicon-based nanowires 120 during
charging and discharging.
[0060] The silicon-based nanowires 120 are disposed on a surface of
the carbonaceous core 110. In this regard, the term "silicon-based"
used herein refers to inclusion of at least about 50 wt % of
silicon (Si), for example, at least about 60 wt %, about 70 wt %,
about 80 wt %, or about 90 wt % of Si, or may include only 100 wt %
of Si. Also, in this regard, the term "nanowire" used herein refers
to a wire structure having a nano-diameter cross-section. For
example, the nanowire may have a cross-section diameter of about 10
to about 500 nm and a length of about 0.1 to about 100 .mu.m. Also,
an aspect ratio (length:width) of each nanowire may be 10 or more,
for example, 50 or more, or for example, 100 or more. Also,
diameters of nanowires may be substantially identical to or
different from each other, and from among longer axes of nanowires,
at least a portion may be linear, gently or sharply curved, or
branched. Such silicon-based nanowires may withstand a volumetric
change of a lithium battery due to charging and discharging.
[0061] The silicon-based nanowires 120 may include, for example, at
least one of Si, SiOx (0<x.ltoreq.2), and Si--Z alloys (where Z
is an alkali metal, an alkali earth metal, a Group 13 element, a
Group 14 element, a transition metal, a rare earth element, or a
combination thereof and is not Si), but a material for forming the
silicon-based nanowires 120 is not limited thereto. The element Z
may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra,
Sc, Y, La, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, W, Tc, Re, Fe, Ru, Os,
Co, Rh, Ir, Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Ge, P, As, Sb, Bi,
S, Se, Te, Po, and combinations thereof. Also, Si, SiO.sub.x, and
the alloy of Si and Z may include amorphous silicon, crystalline
(including single or poly crystalline) silicon, or a combination
thereof. The silicon-based nanowires 120 may include these
materials alone or in a combination. For example, Si nanowires may
be used as the silicon-based nanowires 120 in consideration of high
capacity.
[0062] The silicon-based nanowires 120 may be manufactured by
directly growing silicon-based nanowires on the carbonaceous core
110, or by disposing, for example, attaching or coupling
silicon-based nanowires which have been grown separately to the
carbonaceous core 110. The silicon-based nanowires 120 may be
disposed on the carbonaceous core 110 by using any known placing
methods. For example, a nanowire may be grown by using
vapor-liquid-solid (VLS) growth method, or using a nano-sized
catalyst that thermally decomposes a precursor gas present nearby.
The silicon-based nanowires 120 may be directly grown on the
carbonaceous core 110 in the presence or absence of a metal
catalyst. Examples of the metal catalyst are Pt, Fe, Ni, Co, Au,
Ag, Cu, Zn, Cd, etc.
[0063] According to an embodiment of the present invention, based
on the total amount of the crystalline carbonaceous core 110 and
the silicon-based nanowires 120, an amount of the crystalline
carbonaceous core 110 may be in a range of about 60 to about 99 wt
% and an amount of the silicon-based nanowires 120 may be in a
range of about 1 to about 40 wt %. Due to the inclusion of this
amount range of high-capacity silicon-based nanowires, a
high-capacity negative active material may be obtained.
[0064] The amorphous carbonaceous coating layer 130 is coated on
the crystalline carbonaceous core 110 with the silicon-based
nanowires 120 disposed on a surface thereof so as not to expose at
least a portion of the silicon-based nanowires 120. The term
"amorphous" refers to a case in which a distinctive crystal
structure is not present. The amorphous carbonaceous coating layer
130 may include, for example, at least about 50 wt %, about 60 wt
%, about 70 wt %, about 80 wt %, or about 90 wt % of amorphous
carbon, or may include 100 wt % of amorphous carbon.
[0065] According to an embodiment of the present invention, a D/G
ratio of the amorphous carbonaceous coating layer 130 may be 3.0 or
more, wherein the D/G ratio is a ratio of a D (defect) band peak
intensity area with respect to a G (graphite) band peak intensity
area in a Raman spectrum. For example, in the Raman spectrum, the
D/G ratio of the amorphous carbonaceous coating layer 130 may be in
a range of 3.0 to 4.0, for example, 3.1 to 3.6, 3.1 to 3.2, or 3.3
to 3.6. These D/G ratio values are distinguished from those of the
crystalline carbonaceous core 110.
[0066] According to an embodiment of the present invention, the
amorphous carbonaceous coating layer 130 may be formed in such a
way that at least 50 vol % of the silicon-based nanowires 120 are
embedded in the amorphous carbonaceous coating layer 130. For
example, at least 60 vol %, 70 vol %, 80 vol %, or 90 vol % of the
silicon-based nanowires 120 are embedded in the amorphous
carbonaceous coating layer 130, or the silicon-based nanowires 120
may be completely embedded not to be exposed to a surface of the
primary particle.
[0067] The amorphous carbonaceous coating layer 130 prevents (or
protects from) separation or elimination of the silicon-based
nanowires 120 during charging and discharging, thereby contributing
to stability of an electrode and an increase of a lifespan of an
electrode. Also, the amorphous carbonaceous coating layer 130 may
provide electric conductivity for the negative active material, of
which an electric conductivity has been reduced due to the
silicon-based nanowires 120, and improve efficiency
characteristics.
[0068] According to an embodiment of the present invention, the
amorphous carbonaceous coating layer 130 may include soft carbon
(cold calcination carbon), hard carbon, pitch carbonized material,
mesophase carbonized material, calcined corks, or a combination
thereof.
[0069] A coating method for the amorphous carbonaceous coating
layer 130 may be, but is not limited to, dry coating or liquid
coating. Examples of the dry coating are deposition, chemical vapor
deposition (CVD), etc, and examples of the liquid coating are
impregnation, spraying, etc. For example, the crystalline
carbonaceous core 110 on which the silicon-based nanowires 120 are
disposed may be coated with a carbon precursor, such as a
coal-based pitch, a mesophase pitch, a petroleum-based pitch, a
coal-based oil, a petroleum-based crude oil, an organic synthetic
pitch, or a polymer resin, such as a phenol resin, a furan resin, a
polyimide resin, or the like, followed by heat treating to form the
amorphous carbonaceous coating layer 130.
[0070] The amorphous carbonaceous coating layer 130 may be formed
in such a thickness that the amorphous carbonaceous coating layer
130 provides a sufficient conductive passage between primary
particles without a decrease in battery capacity. For example, the
thickness of the amorphous carbonaceous coating layer 130 may be in
a range of about 0.1 to about 10 .mu.m, for example, about 0.5 to
about 10 .mu.m, or about 1 to about 5 .mu.m, but is not limited
thereto.
[0071] According to an embodiment of the present invention, an
amount of the amorphous carbonaceous coating layer 130 may be in a
range of about 0.1 to about 30 wt % based on the primary particle.
For example, an amount of the amorphous carbonaceous coating layer
130 may be in a range of about 1 to about 25 wt %, or 5 to 25 wt %,
based on the primary particle. Within the range described above,
the amorphous carbonaceous coating layer 130 may have an
appropriate thickness, and may provide conductivity to a negative
active material.
[0072] According to an embodiment of the present invention, the
primary particle may be agglomerated or combined with each other to
form a secondary particle, or may be combined with other active
components to form a secondary particle.
[0073] According to an embodiment of the present invention, the
negative active material may further include, together with the
primary particles, a carbonaceous particle including at least one
of natural graphite, artificial graphite, expandable graphite,
graphene, carbon black, fullerene soot, carbon nanotubes, and
carbon fiber. In this regard, the carbonaceous particle may be
included in a spherical, tabular, fibrous, tubular, or powder form.
For example, the carbonaceous particle may be added in an intrinsic
form thereof, such as a spherical, tabular, fibrous, tubular, or
powder form, to the negative active material, or may be subjected
to a spheroidizing treatment as described with the carbonaceous
core 110 of the primary particles and then added in a spherical
particle form to the negative active material. If spherical
particles are added, a spherical particle formed of a material that
is identical to or different from the carbonaceous core 110 of the
primary particle may be added.
[0074] A lithium battery according to an embodiment of the present
invention includes a negative electrode including the negative
active material; a positive electrode facing the negative
electrode; and an electrolyte disposed between the negative
electrode and the positive electrode.
[0075] The negative electrode may include the negative active
material. The negative electrode may be manufactured by using
various methods. For example, the negative active material, a
binder, and selectively, a conductive agent are mixed in a solvent
to prepare a negative active material composition, and then the
negative active material composition is molded in a set or
predetermined shape. Alternatively, the negative active material
composition may be applied on a current collector, such as a copper
foil or the like.
[0076] The binder included in the negative active material
composition may aid a bond between the negative active material
and, for example, the conductive agent; and a bond between the
negative active material and the current collector. An amount of
the binder herein may be, based on 100 parts by weight of the
negative active material, in a range of 1 to 50 parts by weight.
For example, the amount of the binder may be in a range of 1 to 30
parts by weight, 1 to 20 parts by weight, or 1 to 15 parts by
weight, based on 100 parts by weight of the negative active
material. Examples of the binder are polyvinylidenefluoride,
polyvinylidenechloride, polybenzimidazole, polyimide,
polyvinylacetate, polyacrylonitrile, polyvinylalcohol,
carboxymethylcellulose (CMC), starch, hydroxypropylcellulose,
regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene,
polyethylene, polypropylene, polystyrene, polymethylmethacrylate,
polyaniline, acrylonitrilebutadienestyrene, phenol resin, epoxy
resin, polyethylenetelethphalate, polytetrafluoroethylene,
polyphenylsulfide, polyamideimide, polyetherimide,
polyethylenesulfone, polyamide, polyacetal, polyphenyleneoxide,
polybutylenetelephthalate, ethylene-propylene-diene terpolymer
(EPDM), sulfonated EPDM, styrene butadiene rubber, fluoride rubber,
various copolymers, and a combination thereof.
[0077] The negative electrode may further include a conductive
agent that is included selectively to provide a conductive passage
to the negative active material to further improve electrical
conductivity. As the conductive agent, any material used in a
typical lithium battery may be used herein. Examples of the
conductive agent are a carbonaceous material such as carbon black,
acetylene black, ketjen black, carbon fiber (for example, a vapor
phase growth carbon fiber), or the like; a metal such as copper,
nickel, aluminum, silver, or the like, each of which may be used in
powder or fiber form; a conductive polymer such as a polyphenylene
derivative; and a mixture thereof. An amount of the conductive
agent may be appropriately controlled. For example, the conductive
agent may be added in such an amount that a weight ratio of the
negative active material to the conductive agent is in a range of
about 99:1 to about 90:10.
[0078] The solvent may be N-methylpyrrolidone (NMP), acetone,
water, or the like. An amount of the solvent may be in a range of
about 1 to about 10 parts by weight based on 100 parts by weight of
the negative active material. In one embodiment, if the amount of
the solvent is within this range, an active material layer is
easily formed.
[0079] Also, the current collector may typically be formed in a
thickness of about 3 to about 500 .mu.m. The current collector is
not particularly limited as long as the current collector does not
cause a chemical change in a battery and has conductivity. Examples
of a material that forms the current collector are copper;
stainless steel; aluminum; nickel' titanium; calcined carbon;
copper and stainless steel that are surface-treated with carbon,
nickel, titanium, silver, or the like; an alloy of aluminum and
cadmium; etc. Also, an uneven micro structure may be formed on the
surface of the current collector to enhance a binding force with
the negative active material. Also, the current collector may be
used in various forms including a film, a sheet, a foil, a net, a
porous structure, a foaming structure, a non-woven structure,
etc.
[0080] The prepared negative active material composition may be
directly coated on a current collector to form a negative electrode
plate. Alternatively, the negative active material composition may
be cast onto a separate support and then the negative active
material film separated from the support is laminated on the
current collector, such as a copper foil, to obtain the negative
electrode plate.
[0081] The negative active material composition may be printed on a
flexible electrode substrate to manufacture a printable battery, in
addition to the use in manufacturing a lithium battery.
[0082] Separately, for the manufacture of a positive electrode, a
positive active material composition prepared by mixing a positive
active material, a conductive agent, a binder, and a solvent is
prepared.
[0083] As the positive active material, any lithium-containing
metal oxide that is conventionally used in the art is used herein.
For example, LiCoO.sub.2, LiMn.sub.xO.sub.2x (where x is 1 or 2),
LiNi.sub.1-xMn.sub.xO.sub.2 (where 0<x<1), or
LiNi.sub.1-x-yCo.sub.xMn.sub.yO.sub.2 (where 0.ltoreq.x.ltoreq.0.5
and 0.ltoreq.y.ltoreq.0.5), or the like may be used. For example, a
compound that intercalates and/or deintercalates lithium, such as
LiMn.sub.2O.sub.4, LiCoO.sub.2, LiNiO.sub.2, LiFeO.sub.2,
V.sub.2O.sub.5, TiS, MoS, or the like, may be used as the positive
active material.
[0084] The conductive agent, the binder, and the solvent included
in preparing the positive active material composition may be
identical to those included in the negative active material
composition. In some cases, a plasticizer may be further added to
the positive active material composition and the negative active
material composition to form pores in a corresponding electrode
plate. Amounts of the positive active material, the conductive
agent, the binder, and the solvent may be the same as used in a
conventional lithium battery.
[0085] A positive electrode current collector may have a thickness
of about 3 to about 500 .mu.m, and may be any of various current
collectors that do not cause a chemical change in a battery and has
high conductivity. Examples of the positive electrode current
collector are stainless steel, aluminum, nickel, titanium, calcined
carbon, and aluminum and stainless steel that are surface-treated
with carbon, nickel, titanium, silver, or the like. The positive
electrode current collector may have an uneven micro structure at
its surface to enhance a binding force with the positive active
material. Also, the current collector may be used in various forms
including a film, a sheet, a foil, a net, a porous structure, a
foaming structure, a non-woven structure, etc.
[0086] The prepared positive active material composition may be
directly coated on the positive electrode current collector to form
a positive electrode plate, or may be cast onto a separate support
and then a positive active material film, such as a copper foil,
separated from the support is laminated on the positive electrode
current collector to obtain a positive electrode plate.
[0087] The positive electrode may be separated from the negative
electrode by a separator, and the separator may be any of various
suitable separators that are typically used in a lithium battery.
For example, the separator may include a material that has a low
resistance to migration of ions of an electrolyte and an excellent
electrolytic solution-retaining capability. For example, the
separator may include a material selected from the group consisting
of glass fiber, polyester, Teflon, polyethylene, polypropylene,
polytetrafluoroethylene (PTFE), and a combination thereof, each of
which may be nonwoven or woven. The separator may have a pore size
of about 0.01 to about 10 .mu.m and a thickness of about 5 to about
300 .mu.m.
[0088] A lithium salt-containing non-aqueous based electrolyte
includes a non-aqueous electrolyte and lithium. Examples of the
non-aqueous electrolyte are a non-aqueous electrolytic solution, an
organic solid electrolyte, an inorganic solid electrolyte, etc.
[0089] As the non-aqueous electrolytic solution, a non-protogenic
organic solvent may be used, and examples of the non-protogenic
organic solvent are N-methyl-2-pyrrolidone, propylene carbonate,
ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl
carbonate, fluorinated ethylenecarbonate,
ethylenemethylenecarbonate, methylpropylcarbonate, ethylpropanoate,
methylacetate, ethylacetate, propylacetate, dimethylester
gamma-butyloractone, 1,2-dimethoxy ethane, tetrahydrofurane,
2-methyl tetrahydrofurane, dimethylsulfoxide, 1,3-dioxolane,
formamide, dimethylformamide, acetonitrile, nitromethane, methyl
formic acid, methyl acetatic acid, phosphoric acid trimester,
trimethoxy methane, dioxolane derivatives, sulfolanes, methyl
sulfolanes, 1,3-dimethyl-2-imidazolidinone, propylene carbonate
derivatives, tetrahydrofurane derivatives, ethers, methyl
pyrropionic acid, ethyl pyrropionic acid, etc.
[0090] Examples of the organic solid electrolyte are a polyethylene
derivative, a polyethylene oxide derivative, a polypropylene oxide
derivative, a phosphate ester polymer, polyagitation lysine,
polyester sulfide, polyvinyl alcohol, poly fluorinated vinylidene,
a polymer having an ionic dissociable group, etc.
[0091] Examples of the inorganic solid electrolyte are nitrides,
halides, and sulfides of Li, such as Li.sub.3N, LiI,
Li.sub.5NI.sub.2, Li.sub.3N--LiI--LiOH, LiSiO.sub.4,
Li.sub.2SiS.sub.3, Li.sub.4SiO.sub.4, Li.sub.4SiO.sub.4--LiI--LiOH,
Li.sub.3PO.sub.4--Li.sub.2S--SiS.sub.2, and the like.
[0092] The lithium salt may be any one of various lithium salts
that are suitable for use in a lithium battery. As a material that
is dissolved in the non-aqueous electrolyte, for example, one or
more of LiCl, LiBr, LiI, LiClO.sub.4, LiBF.sub.4,
LiB.sub.10Cl.sub.10, LiPF.sub.6, LiCF.sub.3SO.sub.3,
LiCF.sub.3CO.sub.2, LiAsF.sub.6, LiSbF.sub.6, LiAlCl.sub.4,
CH.sub.3SO.sub.3Li, CF.sub.3SO.sub.3Li,
(CF.sub.3SO.sub.2).sub.2NLi, lithiumchloroborate, lower aliphatic
carbonic acid lithium, 4 phenyl boric acid lithium, imide, etc.,
may be used.
[0093] A lithium battery may be categorized as a lithium ion
battery, a lithium ion polymer battery, or a lithium polymer
battery, according to a separator used and an electrolyte used. A
lithium battery may also be categorized as a cylindrical lithium
battery, a square-shaped lithium battery, a coin-shaped lithium
battery, or a pouch-shaped lithium battery, according to the shape
thereof. A lithium battery may also be categorized as a bulk-size
lithium battery batteries or a thin layer-size lithium battery,
according to the size thereof. The lithium batteries listed above
may also be primary batteries or secondary batteries.
[0094] A method of manufacturing the lithium batteries is apparent
to one skilled in the art and thus will not be described in detail
herein.
[0095] FIG. 2 is a schematic view of a lithium battery 30 according
to an embodiment of the present invention.
[0096] Referring to FIG. 2, the lithium battery 30 includes a
positive electrode 23, a negative electrode 22, and a separator 24
interposed between the positive electrode 23 and the negative
electrode 22. The positive electrode 23, the negative electrode 22,
and the separator 24 are wound or folded to be housed in a battery
case 25. Then, an electrolyte is injected into the battery case 25,
followed by sealing the battery case 25 with an encapsulation
member 26, thereby completing the manufacture of the lithium
battery 30. The battery case 25 may be a cylindrical, rectangular,
or thin film battery case. The lithium battery 30 may be a lithium
ion battery.
[0097] A lithium battery according to an embodiment of the present
invention may be used in, in addition to a mobile phone or a
portable computer, an application, such as an electric vehicle,
that requires high capacity, high power output, and
high-temperature driving. Also, the lithium battery may be combined
with an existing internal-combustion engine, a fuel cell, a super
capacitor, or the like for use in a hybrid vehicle, or the like.
Furthermore, the lithium battery may be used in any other suitable
applications that require high power output, high voltage, and
high-temperature driving.
[0098] Hereinafter, exemplary embodiments of the present invention
will be described in detail with reference to examples. However,
the examples are illustrated for illustrative purposes only and do
not limit the scope of the present invention.
EXAMPLE 1
[0099] Si nanowires (SiNWs) were grown on spherical natural
graphite (Hitachi Chemical Company) having an average diameter of
about 10 .mu.m by vapor-liquid-solid (VLS) growth. Then, the
spherical graphite particles were randomly collected and then a
circularity thereof was measured by using FPIA-3000. The
circularity was in a range of 0.808 to 1.000. The grown SiNWs had
an average diameter of about 30 to about 50 nm, an average length
of about 1.5 .mu.m, and an amount of SiNW was 7.15 wt %.
[0100] 3 wt % of coal tar pitch based on 100 w % of the entire
active material was coated on a surface of the spherical graphite
having the grown SiNWs thereon. The pitch coated spherical graphite
was heat treated at the temperature of 800.degree. C. in a nitrogen
atmosphere to complete the preparation of a negative active
material.
[0101] The prepared negative active material and LSR7 (a
manufacturer: Hitachi Chemical, a binder that consists of 23 wt %
of PAI and 97wt % N-methyl-2-pyrrolidone) as a binder were mixed in
a weight ratio of 90:10 and then N-methylpyrrolidone was added
thereto to control the viscosity thereof until a solid content
thereof reached 30 to 50 wt %, thereby completing preparation of a
negative active material slurry. The prepared slurry was coated on
a copper foil current collector having a thickness of 10 .mu.m to
manufacture a negative electrode plate. The completely coated
electrode plate was dried at the temperature of 120.degree. C. for
15 minutes, followed by pressing, thereby completing the
manufacture of a negative electrode having a thickness of 60 .mu.m.
An Li metal as a reference electrode and a polyethylene separator
having a thickness of 20 .mu.m (product name: STAR20, Asahi) were
used, and an electrolyte was injected thereto, and the resultant
structure was pressed to complete the manufacture of a 2016R type
coin cell. In this case, the electrolyte was 0.75 M LiPF.sub.6
dissolved in a mixed solvent of ethylene carbonate (EC), diethyl
carbonate (DEC), ethylpropanoate (EP), and fluoro ethylene
carbonate (FEC) at a volumetric ratio of
EC:DEC:EP:FEC=25.3:3:40.7:38:8. After welding a coin cell, vacuum
drying was performed thereon at the temperature of 160.degree. C.
for 2 hours to harden the binder and remove moisture.
EXAMPLE 2
[0102] A negative active material and a coin cell were prepared in
the same manner as in Example 1, except that in preparing the
negative active material, 6 wt % of coal tar pitch based on 100 w %
of the entire active material was used for pitch coating.
EXAMPLE 3
[0103] A negative active material and a coin cell were prepared in
the same manner as in Example 1, except that in preparing the
negative active material, 10 wt % of coal tar pitch based on 100 w
% of the entire active material was used for pitch coating.
EXAMPLE 4
[0104] A negative active material and a coin cell were prepared in
the same manner as in Example 1, except that in preparing the
negative active material, 15 wt % of coal tar pitch based on 100 w
% of the entire active material was used for pitch coating.
COMPARATIVE EXAMPLE 1
[0105] A coin cell was prepared in the same manner as in Example 1,
except that primary particles prepared by growing SiNWs on
spherical graphite were used as a negative active material without
pitch coating.
(Negative Active Material Analysis)
EVALUATION EXAMPLE 1
Analysis on Field Emission Scanning Electron Microscope (FE-SEM)
Image of Negative Active Material
[0106] The negative active materials used in preparing the coin
cells according to Example 1 and Comparative Example 1 were
analyzed by FE-SEM. FE-SEM images of a cross section of the
negative active material used in Example 1 are shown in FIGS. 3A
and 3B. FE-SEM images of a cross section of the negative active
material used in Comparative Example 1 are shown in FIGS. 4A and
4B.
[0107] As shown in FIGS. 4A and 4B, regarding the negative active
material used in Comparative Example 1, SiNWs grown in spherical
graphite are exposed. However, as shown in FIGS. 3A and 3B,
regarding the negative active material used in Example 1, a pitch
coating layer having a thickness of about 1.5 to 2.mu.m is formed
on spherical graphite on which SiNWs are grown and the pitch
coating layer surrounds SiNWs.
EVALUATION EXAMPLE 2
Raman Spectrum Analysis of Negative Active Material
[0108] Raman spectrum analysis was performed on a graphite core and
a pitch coating layer included in the negative active material used
in manufacturing the coin cell of Example 1, and results thereof
are shown in FIG. 5.
[0109] Raman spectrum analysis on the graphite core and the pitch
coating layer were repeatedly performed three times, and the D/G
ratio defined as Equation 1 below was calculated and results
thereof are shown in Table 1 below.
D/G ratio=[Intensity area of D band peak]/[Intensity area of G band
peak] Equation 1
[0110] In a Raman spectrum, a D band peak may have its peak center
in a frequency number of 1340 to 1360 cm.sup.-1, and a G band peak
may have its peak center in a frequency number of 1570 to 1590
cm.sup.-1.
TABLE-US-00001 TABLE 1 D/G Measuring Measuring Measuring once twice
three times Graphite (core) 0.1 0.1 0.1 Pitch (coating layer) 3.2
3.1 3.2
[0111] As shown in Table 1, the D/G ratio of the graphite core was
about 0.1, and the D/G ratio of the pitch coating layer was in a
range of 3.1 to 3.2. Such a D/G difference may be due to different
crystallinity of the graphite core and the pitch coating layer.
EVALUATION EXAMPLE 3
Particle Distribution Analysis of Negative Active Material
[0112] Particle distributions of the negative active materials used
in the coin cells of Example 1 and Comparative Example 1 were
measured by using a Beckmann culter counter particle distribution
analyzer, and results thereof are shown in Table 2 below and FIG.
6.
TABLE-US-00002 TABLE 2 Pitch coating amount D10 D50 D90 Comparative
0 wt % 0.17 11.6 18.2 Example 1 Example 1 3 wt % 7.61 12.7 16.9
Example 2 6 wt % 8.08 12.9 19.2 Example 3 10 wt % 7.96 13.3 23.9
Example 4 15 wt % 8.19 15.0 35.1 [unit: .mu.m]
[0113] As shown in Table 2 and FIG. 6, it was confirmed that in the
case of the negative active material of Comparative Example 1 in
which pitch coating was not performed, SiNWs were separated at a
particle size of 1 .mu.m or less, and in the case of the negative
active materials of Example 1-3 in which pitch coating was
performed, SiNWs were not separated at a particle size of 1 .mu.m
or less, and a particle size was increased due to the pitch
coating.
EVALUATION EXAMPLE 3
Electric Conductivity of Negative Active Material
[0114] To measure electric conductivity of the negative active
materials used in manufacturing the coin cells of Example 1 and
Comparative Example 1, an electric conductivity evaluator
(MCP-PD51, Mitsubishi Chemical Company) was used to measure
electric conductivity of powder with respect to a pressed
density.
[0115] The negative active materials used in manufacturing the coin
cells of Example 1 and Comparative Example 1 were filled in holders
and then pressure was applied thereto to prepare pellets. A mass of
each of the pellets was 2.040 g. A distance between electrodes was
3 mm, a radius of an electrode was 0.7 mm, and a radius of each
pellet was 10 mm. At each pressure, a resistance (R) of a pattern
was measured by using a four-point probe. A specific resistance and
an electric conductivity were measured using correction factors in
consideration of the thickness and shape of the pattern and the
resistance.
[0116] Specific resistance measurement formulation:
.rho.=G.times.R, G=3.575.times.t (.rho.: specific resistance, R:
resistance, G: shape correction factor, and t: pattern
thickness)
.sigma. = 1 .rho. ##EQU00001##
[0117] .sigma.: electric conductivity, .rho.: specific
resistance
[0118] The electric conductivity evaluation results are shown in
FIG. 7. Referring to FIG. 7, the greater the pitch coating amount
is, the higher the electric conductivity is. The increased electric
conductivity may contribute to improvement in efficiency and
lifetime characteristics.
EVALUATION EXAMPLE 4
pH of Negative Active Material
[0119] To evaluate pH of the negative active materials used in
manufacturing the coin cells of Examples 1 to 4 and Comparative
Example 1, 5 wt % negative active material solution was prepared
using deionized (DI) water and then stirred and left to sit for 30
minutes. In the solution, graphite was sunk and separated from the
SiNWs that were floated. A pH of the solution was measured. The pH
data are shown in FIG. 8.
[0120] As shown in FIG. 8, the greater the pitch coating amount is,
the higher the pH is. SiNWs are acidic so that gelation occurs when
a slurry is prepared. However, when pitch coating is performed as
in the examples above, SiNWs are neutralized to prevent the slurry
gelation and improve proccessability.
(Cell Properties Evaluation)
EVALUATION EXAMPLE 5
Electrode Volumetric Expansion Ratio Measurement
[0121] The coin cells of Examples 1-4 and Comparative Example 1
were charged (formation) at a current of 0.05 C and then the coin
cells were disassembled to compare a thickness of a negative
electrode plate before and after the charging, and a volumetric
expansion ratio of the negative electrodes of the coin cells was
measured. The results thereof are shown in FIG. 9.
[0122] As shown in FIG. 9, the greater the pitch coating amount is,
the less the expansion ratio is. This is because SiNWs suppress the
expansion. The decreased expansion ratio may contribute to improved
charge and discharge efficiency and lifespan improvement.
EVALUATION EXAMPLE 6
Charging and Discharging Test
[0123] The coin cells of Examples 1-4 and Comparative Example 1
were charged at a current of 40 mA per 1 g of a negative active
material until a voltage reached 0.001 V(vs. Li), and then
discharged with the same amplitude of current until the voltage
reached 3 V (vs. Li). Then, within the same current and voltage
ranges, charging and discharging were repeatedly performed 50
times.
[0124] This charging and discharging test was performed at room
temperature of 25.degree. C. A charge-discharge efficiency (CDE) is
defined according to Equation 2 below. A capacity retention ratio
(CRR) is defined according to Equation 3 below.
CDE [%]=[discharging capacity in each cycle/charging capacity in
the same cycle].times.100 Equation 2
CRR [%]=discharging capacity in a 50th cycle/discharging capacity
in a first cycle Equation 3
[0125] CDE data of the coin cells of Examples 1-4 and Comparative
Example 1 are shown in FIG. 10A, and FIG. 10B is an enlarged view
of an x-axis of FIG. 10A.
[0126] Referring to FIGS. 10A and 10B, the greater the pitch
coating amount is, the higher the charge-discharge efficiency is.
This is because due to the pitch coating, conductivity of an
electrode plate is increased, and also an expansion ratio is
controlled and thus, stability of an electrode plate is guaranteed.
Also, when pitch coating was performed, an irreversible capacity
ratio is reduced and thus, initial efficiency of cycle is
improved.
[0127] Also, CRR data of the coin cells of Examples 1-4 and
Comparative Example 1 are shown in FIG. 11A, and FIG. 11B is an
enlarged view of an x-axis of FIG. 11A. Referring to FIGS. 11A and
11B, the greater the pitch coating amount is, the higher the CRR
is.
[0128] From the charging and discharging results, it is confirmed
that due to the pitch coating on the SiNW negative active material
prepared using spherical graphite as a base material, electric
conductivity of an electrode plate is increased, and also an
expansion ratio is controlled and thus stability of the electrode
plate is guaranteed, and rate characteristics and lifespan
characteristics are improved.
[0129] The negative active material may control an expansion ratio
during charging and discharging of a lithium battery and may
provide conductivity to a negative electrode plate to improve CDE
and cycle lifespan characteristics of a lithium battery.
[0130] It should be understood that the exemplary embodiments
described herein should be considered in a descriptive sense only
and not for purposes of limitation. Descriptions of features or
aspects within each embodiment should typically be considered as
available for other similar features or aspects in other
embodiments. That is, while the present invention has been
described in connection with certain exemplary embodiments, it is
to be understood that the invention is not limited to the disclosed
embodiments, but, on the contrary, is intended to cover various
modifications and equivalent arrangements included within the
spirit and scope of the appended claims, and equivalents
thereof.
* * * * *